1. Technical Field
The present invention relates to a light source having a discharge lamp including a pair of electrodes, a method for driving the light source, and a projector having the light source incorporated therein.
2. Related Art
Since electrodes of a discharge light emitting lamp wears with the lighting time and the distance between the electrodes that influences illuminance of the lamp increases with time, restraining the increase in the distance between electrodes is a problem.
To deal with this problem, in a first technique, a rise in the voltage between the electrodes due to the increase in the distance between the electrodes is monitored. When the voltage value exceeds an upper limit value, the lighting frequency is raised in stages every predetermined time to 300 Hz or higher, as disclosed in JP-A-2005-276623. Thus, a protrusion is reformed on the distal end side of the electrodes. The rise in the voltage, that is, the increase in the distance between the electrodes, is restrained. 
Also, to deal with the above problem, in a second technique, a lighting frequency selected from the range of 60 to 1000 Hz is set as a steady-state lighting frequency, and a low frequency selected within the range of 5 to 200 Hz is inserted with a proper length and in proper timing, as disclosed in JP-A-2006-59790. Thus, the surface on the distal end side of the electrodes is melted and flattened while the main protrusion is left. An arc jump is thus restrained and a protrusion is re-formed.
Moreover, to deal with the above problem, in a third technique, the frequency, and the quantity and timing of a triangle wave to be superimposed are changed in accordance with the voltage or current between the electrodes, as disclosed in Japanese Patent No. 3,934,436. This technique uses a driving waveform on which a triangle wave is superimposed. As the quantity of superimposition of the triangle wave along the rise in the voltage between electrodes, a spot, that is, a protrusion, can be grown to reduce the voltage between the electrodes.
There are other techniques including a driving method in which a change such as arc anomaly is detected from a lamp voltage or the like and a steady-state driving waveform or the like is adjusted, or a driving  method in which a use integration time is monitored and the driving waveform is adjusted in accordance with the use integration time, as disclosed in JP-A-2004-39563, JP-A-2003-264094, JP-A-2005-209572, WO2004/066687, and JP-A-2005-310484.
However, though the method of driving with a high-frequency current can form a protrusion on the distal end side of electrodes relatively quickly, as in the first technique and the second technique, the method has problems that the formed protrusion is weak and more easily moves on the distal end side of the electrodes, and that plural protrusions are formed. As deterioration continues, this further leads to problems including instability of the position of the protrusion formed on the electrodes, flattening of the distal end side of the electrodes due to wear, acceleration of change in the protrusion due to the combined effect of these, acceleration of wear of the distal end side of the electrodes in endurance, and change in the quantity of light intake to an optical system.
Meanwhile, the method of adjusting the quantity of superimposition of a triangle wave in accordance with the voltage between electrodes can grow a protrusion by adjusting the quantity of superimposition of the triangle wave, as in the third technique. However, the quantity  of superimposition of the triangle wave alone does not enable formation of a protrusion having a sufficient size and a sufficiently stable shape. Therefore, this method is only slightly superior to a method using a general rectangular wave, which is traditionally used, or a method of adding a pulse waveform to the rear end of the rectangular wave.
Also in the other techniques, the formed protrusion tends to be weak. A protrusion having a sufficient size and a sufficiently stable shape cannot be formed.